1. Field of the Invention
The present invention relates to a fuel injection control system suitable to inject or spray out fuel, stored in a fuel tank of automotive vehicles such as passenger cars, trucks and the like, through injector valves into each individual engine cylinder, and specifically to a system capable of variably controlling or adjusting a fuel-pressure level of fuel delivered to the injector valves.
2. Description of the Prior Art
Generally, a fuel injection control system for automotive engines is comprised of a common rail equipped with a plurality of injector valves used for fuel delivery to each engine cylinder of an internal combustion engine, a fuel pump connected to the common rail for sucking the fuel into its pressurized chamber on a suction stroke and for discharging the fuel from the pressurized chamber toward the common rail on a discharge stroke, and a fuel-pressure control valve capable of controlling or regulating the fuel pressure in the common rail by relieving the fuel in the pressurized chamber of the fuel pump at a predetermined timing. Such conventional fuel injection systems have been disclosed in Japanese Patent Provisional Publication Nos. 62-258160 and 2-146256. In this type of conventional fuel injection control system, the fuel-pressure control valve is typically constructed by a normally-open type electromagnetically-operated solenoid valve. As is generally known, a fuel-pressure control valve, consisting of a normally-open type, electromagnetic solenoid valve, is shifted to its fully-closed state when energized in response to a command signal from a control unit. On the contrary, when the fuel-pressure control valve is de-energized in response to a command signal (an inoperative signal) from the control unit, the control valve is kept in its inoperative state (or in a full-open state). Usually, the control unit would function to relieve fuel in the pressurized chamber into the fuel tank at the latter-half stage of the discharge stroke, by keeping the fuel-pressure control valve at its closed position for a time duration from the start of the suction stroke to the middle stage of the discharge stroke, and by relieving the fuel in the pressurized chamber of the fuel pump into the fuel tank during the latter-half period of the discharge stroke. The actual discharge stroke of the fuel pump corresponds to a particular time duration from the start of the discharge stroke to a time when the fuel-pressure control valve is shifted to the open position. Actually, the control unit adjusts an amount of fuel discharged from the fuel pump and variably controls the fuel pressure in the common rail depending upon the operating condition of the engine, by stopping energisation of the fuel-pressure control valve and by varying a valve-open timing of the fuel-pressure control valve. In such conventional fuel injection control systems, a fuel-pressure control valve is often constructed by a normally-open type electromagnetic solenoid valve. For example, in case that a signal line provided between the fuel-pressure control valve and the control unit is broken, or when the electromagnetic actuator employed in the fuel-pressure control valve fails, there is a possibility that the fuel-pressure control valve cannot be moved to the closed position. In such cases, that is, owing to the signal-line failure or the fuel-pressure control valve failure, the fuel-pressure control valve remains open. Under this condition, the fuel in the pressurized chamber of the fuel pump would be relieved toward the fuel tank all over the discharge stroke. As set out above, in the conventional fuel injection control systems, when the fuel-pressure control valve cannot be shifted to the closed position owing to troubles such as a signal line failure or a fuel-pressure control valve failure, the fuel cannot be discharged from the fuel pump into the common rail due to fuel relief from the pressurized chamber into the fuel tank with the fuel-pressure control valve unintendedly kept at its full-open position. This results in a drop in fuel pressure of the fuel prevailing in the common rail, and whereby the injection amount of fuel injected from each fuel injector valve tends to become unstable. As previously noted, the conventional fuel injection control system suffers from the drawback that the operating condition of the internal combustion engine deteriorates, when a fuel-pressure control valve, constructed by a normally-open type electromagnetic solenoid valve, cannot be shifted to its closed position due to troubles for example a fuel-pressure control valve failure.